Well, I for one would not advise doing things one handed. With a chainsaw that's probably the best way to get cut. You have to remember it is still a spinning sawblade no matter the size. The big metal thing where you put your second hand is an emergency brake. It is designed for kick back which can and will happen. When it does it is extremely violent and not even the biggest of lumberjacks can stop it. When it happens, it is designed so that your second hand, the one holding on to that handle at the top will mash into that guard. The Guard applies an emergency brake to the chain stopping it before it can cut you. If you use that one handed the kick back can actually kick it and spin the blade around back into your arm, because trust me the human wrist can't stop that.
Please hold the chain saw properly for your own safety. They are designed to be held two handed for a purpose. It may seem small and harmless. If used properly they are as safe as any other saw. However used properly are the key words.
Watch this video, the guy goes through it kind of quickly but he covers the braking system on a chainsaw and how to use it.
https://www.youtube.com/watch?v=3zDCyPLm7To